There are numerous medical devices which are implanted into the body of animals, including humans (hereinafter collectively referred to as "patients"). These devices are resident for periods of time ranging from several days to the lifetime of the patient. Such implanted devices encompass a wide range of applications, including (but not limited to) artificial joints, artificial ligaments, artificial tendons, bone implants, orthotic devices, orthopedic correctional and supporting devices (e.g., screws and braces), shunts, stents, pumps, collection reservoirs, drug delivery depots, temperature sensors, pressure sensors, temporary surgical staples, and the like.
Such implanted devices may be related to one or more changing physical parameters. These changes may be associated with, for example, a feature of the device, a result of a physiological impact on the implanted device, or may be indicative of the success or failure of the implanted device. In this context, representative changing physical parameters include temperature, strain, oscillation, pressure, volume, flow, acceleration, angular momentum, angular velocity, chemical composition, pH, ionic content, changing material characteristics of an anatomical structure or of the implantable, and the like.
There is clinical value in being able to measure certain physical parameters associated with physiological processes and anatomical conditions. This measurement process involves four parts: (1) the sensing of a condition or a changing condition; (2) the transduction of the sensor input to an appropriate energy or signal format; (3) signal conditioning to make the transduced signal suitable for transmission; and (4) the reporting or transmission of the information. There exist systems to address all four parts, though these are usually made of discrete components dedicated to each of the four parts. For example, an implanted thermistor would measure temperature, convert the temperature change into an impedance change, registering a voltage drop, the numerical value of which is transmitted in coded format via dynamic electromagnetic signal. Similarly, an implanted blood pressure monitoring device may consist of an air-filled dome over a silicon-based pressure sensor which converts strain to a change of an electrical parameter. This change is measured, coded, transmitted via high frequency electromagnetic emission. For both examples, a receiver external to the body would register the emitted coded signal, providing decoding, interpretation, and data display.
Certain physiological and anatomical parameters can also be detected by entirely non-invasive means, such as magnetic resonance imaging (MRI), ultrasound, and X-ray techniques. However, these methods have come to rely more and more on the infusion of certain contrast agents in the subject area. Such infusion is an invasive process.
While such sensing and transmission techniques have shown to be effective, there is still a need in the art for improved devices and methods for non-invasively sensing changing physical parameters. The present invention fulfills these needs, and provides further related advantages.